U.S. patent number 5,233,445 [Application Number 07/806,573] was granted by the patent office on 1993-08-03 for active matrix liquid crystal display with average dielectric constant greater than 6 and less than 1.8 meq/g cyano groups.
This patent grant is currently assigned to Raychem Corporation. Invention is credited to Philip J. Jones, Hundi P. Kamath, Stephen S. Moore, Robert H. Reamey, Mark F. Wartenberg.
United States Patent |
5,233,445 |
Kamath , et al. |
August 3, 1993 |
**Please see images for:
( Certificate of Correction ) ** |
Active matrix liquid crystal display with average dielectric
constant greater than 6 and less than 1.8 meq/g cyano groups
Abstract
An active matrix liquid crystal display having improved
performance properties is made using an encapsulated liquid crystal
structure comprising a containment medium having dispersed therein
a liquid crystal composition having an average dielectric constant
greater than about 6 and other specified characteristics.
Inventors: |
Kamath; Hundi P. (Los Altos,
CA), Reamey; Robert H. (Palo Alto, CA), Wartenberg; Mark
F. (San Jose, CA), Moore; Stephen S. (Redwood City,
CA), Jones; Philip J. (Menlo Park, CA) |
Assignee: |
Raychem Corporation (Menlo
Park, CA)
|
Family
ID: |
25194341 |
Appl.
No.: |
07/806,573 |
Filed: |
December 12, 1991 |
Current U.S.
Class: |
349/86;
252/299.01; 349/177; 349/35 |
Current CPC
Class: |
C09K
19/42 (20130101); G02F 1/1334 (20130101); C09K
19/544 (20130101); G02F 1/1362 (20130101) |
Current International
Class: |
C09K
19/54 (20060101); C09K 19/42 (20060101); G02F
1/13 (20060101); G02F 1/1334 (20060101); G02F
1/1362 (20060101); G02F 001/13 (); C09K
019/52 () |
Field of
Search: |
;359/51,52,106
;252/299.01 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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|
|
|
|
|
|
0198168 |
|
Oct 1986 |
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EP |
|
0313053 |
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Apr 1989 |
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EP |
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3-98022 |
|
Apr 1991 |
|
JP |
|
WO90/03593 |
|
Apr 1990 |
|
WO |
|
WO91/09092 |
|
Jun 1991 |
|
WO |
|
WO91/10716 |
|
Jul 1991 |
|
WO |
|
WO91/05029 |
|
Apr 1992 |
|
WO |
|
Other References
Chisso Corporation Brochure "LIXON Information" (Sep. 1990). .
Finkenzeller et al., "Physical Properties of Liquid Crystals: III
Dielectric Permitivities", The Merck Group Liquid Crystal
Newsletter, No. 4, (Mar. 1989). .
Yaniv et al., "Active Matrix Polymer Dispersed Liquid Crystal
Display", Japan Display '89 (1989), pp. 572-575. .
Weber et al., "Liquid crystals for active matrix displays", Liquid
Crystals, vol. 5, No. 5 (1989), pp. 1381-1388. .
Finkenzeller et al., "Physical Properties of Liquid Crystals: IV.
Optical Aristrophy" The Merck Group Liquid Crystal Newsletter, No.
5 (Oct. 1989), pp. 1-5. .
Plach et al., "Liquid-Crystal Mixtures for Active-Matrix Displays
Using New Terminally Fluorinated Compounds", SID '90 Digest (1990),
pp. 91-94. .
Hirai et al., "Liquid Crystal/Polymer Composite Devices for
Active-Matrix Projection Displays", SID '91 Digest (1991), pp.
584-597. .
Wu et al., "Angular discrimination of light transmission through
polymer-dispersed liquid-crystal films", J. Appl. Phys., vol. 62,
No. 9 (1987), pp. 3925-3931..
|
Primary Examiner: Sikes; William L.
Assistant Examiner: Gross; Anita Pellman
Attorney, Agent or Firm: Burkard; Herbert G. Chao; Yuan
Claims
What is claimed is:
1. A liquid crystal display comprising (a) plural pixels comprising
an encapsulated liquid crystal structure comprising a containment
medium having dispersed therein a liquid crystal composition which
(i) contains less than 1.8 meq/g cyano groups and (ii) has an
average dielectric constant .epsilon..sub.avg greater than 6, and
(b) a non-linear switching element connected in series with each
pixel.
2. A liquid crystal display according to claim 1, wherein
.epsilon..sub.avg is between about 6 and about 10.
3. A liquid crystal display according to claim 1, wherein the
liquid crystal composition has an optical anisotropy .DELTA.n
greater than 0.15.
4. A liquid crystal display according to claim 1, wherein the
liquid crystal composition has a threshold voltage V.sub.th less
than 1.8 volt.
5. A liquid crystal display according to claim 1, wherein the
non-linear switching element is a thin film transistor.
6. A liquid crystal display according to claim 1, wherein the
liquid crystal composition is substantially free of cyano
groups.
7. A liquid crystal display according to claim 1, wherein the
liquid crystal composition is operationally nematic and has a
positive dielectric anisotropy.
8. A liquid crystal display according to claim 1, wherein the
liquid crystal composition further comprises a pleochroic dye.
9. A liquid crystal display according to claim 1, wherein the
liquid crystal composition contains more than 2.0 meq/g
fluorine.
10. A liquid crystal display comprising (a) plural pixels
comprising an encapsulated liquid crystal structure comprising a
containment medium having dispersed therein a liquid crystal
composition which has (i) a threshold voltage V.sub.th less than
1.8 volt, (ii) an optical anisotropy .DELTA.n greater than 0.15,
and (iii) an average dielectric constant .epsilon..sub.avg between
about 6 and about 10, and (b) a non-linear switching element
connected in series with each pixel.
11. A liquid crystal display according to claim 10, wherein the
liquid crystal composition contains less than 1.8 meq/g cyano
groups.
12. A liquid crystal display according to claim 10, wherein the
liquid crystal composition is substantially free of cyano
groups.
13. A liquid crystal display according to claim 10, wherein the
non-linear switching element is a thin film transistor.
14. A liquid crystal display according to claim 10, wherein the
liquid crystal composition is operationally nematic.
15. A liquid crystal display according to claim 10, wherein the
liquid crystal composition further comprises a pleochroic dye.
16. A liquid crystal display according to claim 10, wherein the
liquid crystal composition contains more than 2.0 meq/g
fluorine.
17. An encapsulated liquid crystal structure comprising a
containment medium having dispersed therein a liquid crystal
composition which (i) contains less than 1.8 meq/g cyano groups and
(ii) has an average dielectric constant .epsilon..sub.avg greater
than 6.
18. An encapsulated liquid crystal structure according to claim 17,
wherein .epsilon..sub.avg is between about 6 and about 10.
19. An encapsulated liquid crystal structure according to claim 17,
wherein the liquid crystal composition has an optical anisotropy
.DELTA.n greater than 0.15.
20. An encapsulated liquid crystal structure according to claim 17,
wherein the liquid crystal composition has a threshold voltage
V.sub.th less than 1.8 volt.
21. An encapsulated liquid crystal structure according to claim 17,
wherein the liquid crystal composition is substantially free of
cyano groups.
22. An encapsulated liquid crystal structure according to claim 17,
wherein the liquid crystal composition is operationally nematic and
has a positive dielectric anisotropy.
23. An encapsulated liquid crystal structure according to claim 17,
wherein the liquid crystal composition further comprises a
pleochroic dye.
24. An encapsulated liquid crystal display according to claim 17,
wherein the liquid crystal composition contains more than 2.0 meq/g
fluorine.
25. An encapsulated liquid crystal structure comprising a
containment medium having dispersed therein a liquid crystal
composition which has (i) a threshold voltage V.sub.th less than
1.8 volt, (ii) an optical anisotropy .DELTA.n greater than 0.15,
and (iii) an average dielectric constant .epsilon..sub.avg between
about 6 and about 10.
26. An encapsulated liquid crystal structure according to claim 25,
wherein the liquid crystal composition contains less than 1.8 meq/g
cyano groups.
27. An encapsulated liquid crystal structure according to claim 25,
wherein the liquid crystal composition is substantially free of
cyano groups.
28. An encapsulated liquid crystal structure according to claim 25,
wherein the liquid crystal composition is operationally
nematic.
29. An encapsulated liquid crystal structure according to claim 25,
wherein the liquid crystal composition further comprises a
pleochroic dye.
30. An encapsulated liquid crystal display according to claim 25,
wherein the liquid crystal composition contains more than 2.0 meq/g
fluorine.
Description
TECHNICAL FIELD OF THE INVENTION
This invention relates to active matrix liquid crystal displays and
liquid crystal structures for the same.
BACKGROUND OF THE INVENTION
Liquid crystal displays ("LCD's"), in which the electro-optically
active element comprises liquid crystalline material, are well
known in the art.
Where an LCD is used to depict simple figures or alphanumeric
characters, for example numbers via the familiar seven-segment,
figure-eight pattern found in calculators and watches, it is
feasible to directly address each pixel in the display--that is, to
provide each pixel with its own set of electrode leads. But where
the display must depict complex images such as graphics or video
images, a large number of pixels is required, and it becomes
impractical to directly address each one. A display having pixels
arranged in M rows and N columns has M.times.N pixels, thus
requiring M.times.N sets of individual leads for direct addressing.
As the pixel density and/or the size of the display increases, this
number rapidly becomes unmanageable.
Multiplexing provides a method of addressing each pixel, but with a
much lesser number of electrode leads. In its most elementary form,
multiplexing uses a set of M row electrodes in combination with a
set of N column electrodes. By applying the proper electrical
signals to, for example, the 5th row and 8th column electrodes, the
pixel at the 5th row and 8th column can be switched on and off. In
this way, the number of electrode leads can be reduced from
M.times.N to M+N. However, in this form of multiplexing, adjacent
pixels are not independent of each other. When a voltage sufficient
to switch the 5th row-8th column pixel is applied, adjacent pixels
(e.g., the 4th row-8th column pixel) also experience a substantial
voltage and can be inadvertently switched, at least in part,
leading to cross-talk between adjacent pixels.
One type of multiplexed LCD is an active matrix LCD, in which each
pixel is driven (switched from one visual state to another) by an
active switching element such as a thin film transistor ("TFT"),
varistor, diode or MIM. The switching element helps eliminate
cross-talk and maintain an initially applied voltage across the
corresponding pixel, even when it is not being actively addressed,
so that the pixel stays "on" while other pixels are addressed. The
longer the pixels holds the initially applied voltage, the longer
it can be maintained in the "on" state until it is next addressed,
permitting the construction of displays having a larger number of
pixels. If the matrix contains a sufficiently large number of
switching elements of sufficiently small size, high resolution
displays are possible. Active matrix displays are important for
television, computer, and instrument screens.
One type of liquid crystal display employs an encapsulated liquid
crystal structure, in which a liquid crystal composition is
encapsulated or dispersed in a containment medium such as a
polymer. When a voltage corresponding to a sufficiently strong
electric field is applied across the encapsulated liquid crystal
structure (the "field-on" condition), the alignment of the liquid
crystal molecules therein is re-oriented in accordance with the
field, so that incident light is transmitted. Conversely, in the
absence of such a voltage (the "field-off" condition) the alignment
of the liquid crystal molecules is random and/or influenced by the
liquid crystal-matrix interface, so that the structure scatters
and/or absorbs incident light. The applied voltage at which the
structure changes from its field-off condition to its field-on
condition is generally referred to as the threshold voltage.
High quality commercially practical encapsulated liquid crystal
active matrix LCD's make rigorous demands of the encapsulated
liquid crystal structure and the liquid crystal composition
therein. The encapsulated liquid crystal structure must have a high
charge holding ratio, both as made and after use under various
environmental conditions. The threshold voltage must be low, to be
compatible with active matrix capability. Finally, it must have
high scattering performance in the field-off condition to provide
high contrast ratios.
Many encapsulated liquid crystal structures have been proposed for
use in LCD's generally, and some for active matrix displays
specifically. A liquid crystal structure which may be suitable for
a watch or calculator LCD often will not be suitable for an active
matrix LCD. The development of encapsulated liquid crystal
materials for active matrix displays has been a difficult
proposition.
Some disclosures of liquid crystal compositions asserted to be
suitable for encapsulated liquid crystal structures and/or active
matrix displays include: Coates, WO 91/09092 (1991); Coates et al.,
WO 91/05029 (1991); Plach et al., SID 90 Digest, pp. 91-94 (1990);
Plach et al., WO 91/10716 (1991); Kunishima et al., JP Kokai
3-98022 (1991); Chisso Corporation product brochure entitled "LIXON
Information" (Sep. 15, 1990); Weber et al., Liq. Crystals, Vol. 5,
No. 5, pp. 1381-1388 (1989); and Arai et al., EP 0,313,053
(1989).
One of the disclosures (Weber et al.) teaches that a low polarity
or average dielectric constant of the liquid crystal is important
to attain high resistivity liquid crystal compositions.
Specifically taught embodiments have average dielectric constants
of 5.8 or less. However, we have found that such liquid crystal
compositions lead to displays having unsatisfactory contrast ratios
and/or undesirably high threshold voltages.
In other instances, practitioners in the art have used high
dielectric constant liquid crystal materials in encapsulated liquid
crystal displays. However, such liquid crystal materials comprise
substantial amounts of molecules having cyano groups therein,
resulting in displays having poor long-term stability.
SUMMARY OF THE INVENTION
We have unexpectedly discovered that, contrary to the teachings in
the prior art, an active matrix encapsulated liquid crystal display
having superior viewing properties and long-term stability is
achieved by using a liquid crystal composition having a relatively
high average dielectric constant, a minimal content of cyano
groups, and/or other specified characteristics.
Accordingly, this invention provides a liquid crystal display
comprising (a) plural pixels comprising an encapsulated liquid
crystal structure comprising a containment medium having dispersed
therein a liquid crystal composition which contains less than 1.8
meq/g cyano groups and has an average dielectric constant
.epsilon..sub.avg greater than 6 and (b) a non-linear switching
element connected in series with each pixel.
This invention also provides an encapsulated liquid crystal
structure comprising a containment medium having dispersed therein
a liquid crystal composition which contains less than 1.8 meq/g
cyano groups and has an average dielectric constant
.epsilon..sub.avg greater than 6.
In another aspect, this invention provides a liquid crystal display
comprising (a) plural pixels comprising an encapsulated liquid
crystal structure comprising a containment medium having dispersed
therein a liquid crystal composition which has (i) a threshold
voltage V.sub.th less than 1.8 volt, (ii) an optical anisotropy
.DELTA.n greater than 0.15, and (iii) an average dielectric
constant .epsilon..sub.avg greater than 6, and (b) a non-linear
switching element connected in series with each pixel.
In yet another aspect, this invention provides an encapsulated
liquid crystal structure comprising a containment medium having
dispersed therein a liquid crystal composition which has (i) a
threshold voltage V.sub.th less than 1.8 volt, (ii) an optical
anisotropy .DELTA.n greater than 0.15, and (iii) an average
dielectric constant .epsilon..sub.avg greater than 6.
BRIEF DESCRIPTION OF THE DRAWING(s)
FIGS. 1 and 2 show schematically the operation of an LCD comprising
an encapsulated liquid crystal structure.
FIGS. 3 and 3a show an LCD comprising encapsulated liquid crystal
structure of this invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Encapsulated liquid crystal structures and their preparation are
disclosed in U.S. Pat. Nos. 4,435,047 (1984), 4,606,611 (1986),
4,616,903 (1986), and 4,707,080 (1987), all to Fergason; published
European patent application EP 156,615 (1985), by Pearlman et al.;
U.S. Pat. No. 4,671,618 (1987), to Wu et al.; U.S. Pat. Nos.
4,673,255 (1987) and 4,685,771 (1987), to West et al.; U.S. Pat.
No. 4,688,900 (1987) to Doane et al.; and published European patent
application EP 0,313,053 (1989), by Dainippon Ink and Chemicals;
the disclosures of each which are incorporated herein by reference.
In an encapsulated liquid crystal structure, discrete volumes of a
liquid crystal composition are encapsulated, dispersed, embedded or
otherwise contained in a containment medium or matrix. The volumes
are not necessarily limited to spherical or substantially spherical
ones. They may be irregularly shaped, and even interconnected. The
amount of interconnection between volumes may be to an extent such
that the liquid crystals appear to form a continuous phase, as
described in the aforementioned EP 0,313,053. "Liquid crystal
composition" denotes a composition having liquid crystalline
properties, whether that composition consists of a single discrete
liquid crystalline compound, a mixture of different liquid
crystalline compounds, or a mixture of liquid crystalline and
non-liquid crystalline compounds. Preferably, the liquid crystal
composition is nematic or operationally nematic. More preferably,
it also has a positive dielectric anisotropy.
Individual liquid crystal molecules typically have elongated
shapes, with a tendency to align or orient themselves with their
long molecular axes parallel to each other. This alignment causes a
liquid crystal composition to be anisotropic, meaning that its
measured physical, optical, and other properties are dependent on
the direction of measurement (parallel or perpendicular to the
direction of alignment). Further, the alignment direction can be
influenced by an external stimulus, such as an electrical or
magnetic field, causing the liquid crystal composition to exhibit a
particular value of a physical characteristic in one direction when
the stimulus is absent, but rapidly switching to a different value
when the stimulus is applied. It is because of this anisotropy and
its ready realignment that liquid crystal compositions are useful
as materials for displays.
The containment medium for encapsulated liquid crystal structures
is preferably a polymeric material. Suitable containment media
include but are not limited to poly(vinyl alcohol) and its
copolymers, gelatin, polyurethane, poly(ethylene oxide), poly(vinyl
pyrrolidone), cellulosic polymers, natural gums, acrylic and
methacrylic polymers and copolymers, epoxies, polyolefins, vinyl
polymers, and the like. Poly(vinyl alcohol) is a preferred
containment medium.
An encapsulated liquid crystal structure can be formed by
deposition from an emulsion containing both the containment medium
and the liquid crystal composition or by the evaporation of liquid
from a solution containing both containment medium and the liquid
crystal composition. It can also be formed by making an initially
homogeneous mixture containing both containment medium and liquid
crystal composition at an elevated temperature, then cooling to
phase-separate out liquid crystal volumes contained in the
containment medium. Further, it can be formed by an in-situ
polymerization process, in which the containment medium is
polymerized and simultaneously encapsulates a liquid crystal
composition. The liquid crystal composition need not be entirely
surrounded by the polymer, and may exist as part of a system with
co-continuous phases.
Typically, an encapsulated liquid crystal structure is
substantially non-transparent in the absence of a sufficient
electric field (the "field-off" state) and substantially
transparent in the presence of a sufficient electric field (or
"field-on" state). The electric field induces a change in the
alignment of the liquid crystal molecules in the liquid crystal
composition, in turn causing the encapsulated liquid crystal
structure to switch from a highly light-scattering (and/or
absorbent) state to a highly non-scattering and substantially
transparent state. Generally, it is preferred that the liquid
crystal composition have a positive dielectric anisotropy and that
the ordinary index of refraction of the liquid crystal composition
be matched with the index of refraction of the containment medium,
while the extraordinary index of refraction is substantially
mismatched therewith. The physical principles by which such
encapsulated liquid crystal structures operate are described in
further detail in the aforementioned references, particularly the
patents to Fergason. In those portions of the encapsulated liquid
crystal structure to which a sufficient electric field is applied,
the transition from a non-transparent state to a transparent state
occurs, while adjacent areas to which no electric field has been
applied remain non-transparent.
The principle of operation of an encapsulated liquid crystal
structure is illustrated in FIGS. 1 and 2 (like numerals referring
to like elements). Encapsulated liquid crystal structure 8
comprises a containment medium 10 having distributed therein plural
volumes of liquid crystal composition 11 and is positioned between
top and bottom electrodes 12 and 13, made for example of indium tin
oxide ("ITO") or a thin metal coating. A voltage source 14 is
connected to electrodes 12 and 13, but with switch 15 in the open
position (FIG. 1), no voltage is applied across encapsulated liquid
crystal material 12. Incident light (ray A) is scattered, both
backward (rays a' and a") and forward (b' and b"). When switch 15
is closed (FIG. 2), a voltage is applied across encapsulated liquid
crystal material 8, causing molecules in liquid crystal composition
11 to align their long molecular axes with the field of the applied
voltage. Owing to the match between the index of refraction of
containment medium 10 and the ordinary index of refraction of
liquid crystal composition 11, incident light (ray A) is not
scattered, but is transmitted through encapsulated liquid crystal
structure 8.
Pleochroic dyes have been mixed with liquid crystals to form a
solution therewith. The molecules of pleochroic dyes generally
align with the molecules of liquid crystals, so that the
application of the electric field affects not only the predominant
alignment of the liquid crystals, but also of the pleochroic dye.
As the extent of the absorption of incident light by the pleochroic
dye depends on its orientation relative to the incident light, the
application of an external stimulus to a liquid crystal-pleochroic
dye combination provides an mechanism for the controlled
attenuation of light by absorption. (Thus, as used herein, the term
"liquid crystal composition" also means, in context, a liquid
crystal composition containing pleochroic dye dissolved therein.)
Pleochroic dyes may be used in encapsulated liquid crystal
structures to form colored displays. A display capable of
displaying colored images can be formed by depositing side by side
red, blue, and green pixels made from encapsulated liquid crystal
structures of the corresponding color.
FIG. 3 shows an active matrix LCD according to this invention. A
sandwich 40 comprises encapsulated liquid crystal structure 41
between a first support material 43 coated with a transparent
ground plane electrode 42 (made for example of ITO) and a second
support material 45 (typically glass) having thereon an array 44 of
multiplexed non-linear TFT switching elements. Those skilled in the
art will appreciate that other switching elements, such as
varistors, diodes, and MIM's can also be used. The construction of
the array is shown in greater detail in FIG. 3a, corresponding to a
magnification of the portion of FIG. 3 labeled "A." Each pixel is
defined across each electrode 46 (made for example of ITO). The
application of a voltage across each electrode 46 is controlled by
a switching element 47. In turn, each switching element 47 is
addressed in multiplexed fashion via scan line 49 and data line
48.
The anisotropy of a liquid crystal composition extends to many of
its physical properties. One of these properties is its dielectric
constant (.epsilon.), which has two principal values, one
perpendicular (.epsilon..perp.) to the long molecular axis and one
parallel (.epsilon..parallel.) to the long molecular axis. An
average dielectric constant (.epsilon..sub.avg) can be calculated,
which is conventionally (see, e.g., Weber et al., cited supra) a
weighted average according to the formula: ##EQU1## We have
unexpectedly discovered that for encapsulated liquid crystal
structures for active matrix displays, .epsilon..sub.avg should be
greater than 6 (at 1 kHz and 25.degree. C.). It is especially
preferred that .epsilon..sub.avg is between about 6 and about 10.
Hitherto, the prior art has not recognized the importance of this
parameter in encapsulated liquid crystal structures for active
matrix displays, or has recommended a different, less advantageous
value for .epsilon..sub.avg. Dielectric constants and dielectric
anisotropy may be measured for example by the method described by
Finkenzeller et al. in the paper entitled "Physical Properties of
Liquid Crystals: III. Dielectric Permittivities," in The Merck
Group Liquid Crystal Newsletter, No. 4 (March 1989).
In one aspect of the invention, the liquid crystal composition
should have a low cyano content, by which is meant that less than
1.8 meq/g cyano groups (preferably less than 0.5 meq/g). It is
especially preferred that the liquid crystal composition be
substantially free of cyano groups or consists essentially of
compounds free of cyano groups.
As is common in the art, the liquid crystal composition generally
does not consist of a single discrete compound, but is a mixture of
different liquid crystal compounds. It is preferred that the liquid
crystal composition comprises fluorinated liquid crystal compounds,
such that the composition has a fluorine content greater than 2.0
meq/g, preferably between 2.0 and 10.0 meq/g.
Chlorinated liquid crystal compositions are also suitable.
Accordingly, in an especially preferred embodiment the liquid
crystal composition is substantially free of cyano groups and has a
fluorine content between 2.0 and 10.0 meq/g.
The liquid crystal composition preferably has a large optical
anisotropy .DELTA.n, which is the difference between its ordinary
and extraordinary indices of refraction. A large .DELTA.n leads to
a higher degree of scattering in the field-off state, resulting in
a display with improved contrast. Since the ordinary refractive
index of the liquid crystal material is substantially matched to
the refractive index of the containment medium, a larger .DELTA.n
also means a larger difference between the latter and the
extraordinary refractive index of the liquid crystal material. This
latter difference affects the field-off scattering, with larger
amounts of scattering being associated with larger differences.
Preferably, .DELTA.n is greater than 0.15, more preferably between
0.15 and 0.30. Optical anisotropy may be measured for example by
the method described by Finkenzeller et al. entitled "Physical
Properties of Liquid Crystals: IV. Optical Anisotropy," in The
Merck Group Liquid Crystal Newsletter, No. 5 (October 1989).
The liquid crystal composition also preferably has a threshold
voltage V.sub.th at 25.degree. C. of less than 1.8 volt, more
preferably between 0.8 volt. A lower V.sub.th corresponds to a
display whose pixels can be driven at a lower voltage. By V.sub.th
is meant the voltage at 20.degree. C. at which the liquid crystal
composition causes 90% absorption of incident light when used in a
twisted nematic cell, for a measurement made at right angles to the
cell. For more on V.sub.th and its measurement, see the "LIXON
Information" brochure by Chisso Corporation, cited supra, the
disclosure of which is incorporated herein by reference.
The practice of this invention may be further understood by
reference to the following examples, which are provided by means of
illustration, not limitation.
EXAMPLE 1
An encapsulated liquid crystal structure was made by an emulsion
process using a 50:50 weight/weight mixture of liquid crystals
ZLI-4792 and ZLI-3401, both from Merck (Darmstadt, Germany), with
poly(vinyl alcohol) (Airvol 205, Air Products and Chemicals, King
of Prussia, Pa., USA) as the containment medium.
Liquid crystal ZLI-4792 is cyano-free, while liquid crystal
ZLI-3401 is a partially cyanated, with the result that the overall
composition had a cyano content of 1.55 meq/g. The composition also
had a fluorine content of 2.79 meq/g. The fluorine content was
measured by elemental analysis, while the cyano content was
measured by .sup.13 C NMR. Accurately weighed samples of the liquid
crystal composition and toluene (used as an internal intensity
standard) were placed in a 10 mm NMR tube and dissolved in
CDCl.sub.3. A .sup.13 C(.sup.1 H) gated decoupling experiment was
performed on a Varian XL300 spectrometer with a recycle delay of
400 sec to ensure complete polarization of the .sup.13 C nuclei
between successive scans. The integrated peak intensity of the
methyl group of the toluene (ca. 21 ppm relative to TMS) was used
to calculate the number of equivalents of cyano groups present in
the liquid crystal composition, based on the integrated intensity
of the region between 118 and 119 ppm (assigned to cyano
groups).
The voltage holding ratio of the encapsulated liquid crystal
structure was measured as follows. A sample of encapsulated liquid
crystal structure was mounted between two electrodes and short
voltage pulse (30-300 .mu.sec) was applied. The sample was placed
in an open circuit, allowing the charge to decay through the sample
for a given hold period (15 msec). A pulse of opposite polarity was
then applied, simulating conditions to which the sample would be
subjected during actual service in an active matrix LCD. The
voltage holding ratio is expressed as the percentage of the
originally applied voltage retained at the end of the hold period.
Larger values are more desirable. The voltage of the test generally
is the voltage at which the sample achieves 90% of its maximum
transmission. This voltage will vary somewhat from sample to
sample, and even within a sample, depending on its history. To
evaluate the aging stability of an encapsulated liquid crystal
structure, the voltage holding ratio was measured on the structure
as made and on material after an aging cycle, such as 200 hours at
100.degree. C.
The E.sub.90, a thickness-corrected measure of the operating
voltage, and the contrast ratio of the LCD were measured as
follows: The test LCD cell is powered from 0 volt to above the
saturation voltage in 25 equal increments (pausing 0.5 sec at each
voltage). The transmission at 0 volt (% T.sub.off) and at
saturation (% T.sub.on) are noted. Then the voltage V.sub.90, at
which
is also noted. E.sub.90 is then calculated according to the
formula
where d is the thickness of the sample in microns (.mu.). The
contrast ratio (CR) of the cell is given by the equation
##EQU2##
The voltage holding ratio, contrast ratio, and E.sub.90 of
subsequent samples were also measured as described.
EXAMPLE 2
An encapsulated liquid crystal structure was made following the
procedure of Example 1, except that the liquid crystal was RY1005XX
(Chisso Corporation, Tokyo, Japan). RY1005XX is believed to be a
mixture containing liquid crystal compounds of the following
structures: ##STR1## where R.sub.1 is C.sub.3 H.sub.7 or C.sub.5
H.sub.11 ; R.sub.2 is C.sub.2 H.sub.5, C.sub.3 H.sub.7, or C.sub.5
H.sub.11 ; R.sub.3 is C.sub.2 H.sub.5, C.sub.3 H.sub.7, or C.sub.5
H.sub.11 ; and R.sub.4 is C.sub.2 H.sub.5, C.sub.3 H.sub.7, C.sub.4
H.sub.9, or C.sub.5 H.sub.11. It was substantially free of cyano
groups and had a fluorine content of 3.40 meq/g.
EXAMPLE 3
An encapsulated liquid crystal structure was made following the
procedure of Example 1, except that the liquid crystal was RY1002XX
(Chisso Corporation, Tokyo, Japan). RY1002XX is believed to be a
mixture containing liquid crystal compounds (A), (D) (R.sub.3
=C.sub.2 H.sub.5, C.sub.3 H.sub.7, or C.sub.5 H.sub.11), (E)
(R.sub.4 =C.sub.3 H.sub.7, C.sub.4 H.sub.9, or C.sub.5 H.sub.11)
and additionally liquid crystal compounds of the following
structures: ##STR2## where R.sub.5 is C.sub.5 H.sub.11 or C.sub.7
H.sub.15. It was substantially cyano-free and had a fluorine
content of 3.85 meq/g.
EXAMPLE 4
An encapsulated liquid crystal structure was made following the
procedure of Example 1, except that the liquid crystal was liquid
crystal composition 18193, a chlorinated LC mixture from Merck
(Darmstadt, Germany), believed to be cyano-free.
EXAMPLE 5
An encapsulated liquid crystal structure was made following the
procedure of Example 1, except that the liquid crystal was a 50:50
mixture (weight:weight) of liquid crystal composition 18193 and
ZLI-3401, both from Merck (Darmstadt, Germany), having a cyano
content of 1.55 meq/g.
The following comparative examples A-C not according to this
invention are provided for comparison against the improved
properties obtained by using encapsulated liquid crystal structures
according to our invention.
EXAMPLE A
An encapsulated liquid crystal structure was made following the
procedure of Example 1, except that the liquid crystal was 18282
(British Drug House, Poole, England). 18282 is a liquid crystal
composition having a high cyano content (3.95 meq/g) as wells as
fluorine groups (0.60 meq/g).
EXAMPLE B
An encapsulated liquid crystal structure was made following the
procedure of Example 1, except that the liquid crystal was LIXON
5047 (Chisso Corporation, Tokyo, Japan). LIXON 5047 is believed to
be a fluorinated (5.63 meq/g), cyano-free liquid crystal
composition.
EXAMPLE C
An encapsulated liquid crystal structure was made following the
procedure of Example 1, except that the liquid crystal was PN001
(Rodic, Tokyo, Japan). PN001 is a liquid crystal composition
consisting entirely of cyano-containing compounds (cyano content
4.35 meq/g).
The properties of encapsulated liquid crystal structures and
displays made according to the preceding examples of the invention
and the comparative examples are provided in Table I.
TABLE I ______________________________________ PROPERTIES OF
ENCAPSULATED LIQUID CRYSTAL STRUCTURES Voltage Holding Con-
E.sub.90.sup.d V.sub.th Ratio (%) trast (volt/ Ex.
.epsilon..sub.avg (volt) .DELTA.n As made.sup.a Aged.sup.b
Ratio.sup.c .mu.) ______________________________________ 1 7.42
1.67 0.127 95 (19) 70 6.0 2.59 2 7.33 1.61 0.175 96 (12) 93 7.1
2.60 3 7.2 1.85 0.175 96 (30) 95 12.6 3.63 4 7.87 1.95 0.188 98
(53) 95 15.6.sup.e 5.84 5 9.44 1.63 0.174 98 (31) 47 8.5.sup.f 2.72
A 11.97 1.35 0.244 90 (114) .sup. 34.sup.g 16.5 2.8 B 5.5 1.53 0.10
99 (31) 94 3.5 3.78 C 19.87 1.15 0.254 29 (12) -- 6.5 2.37
______________________________________ .sup.a At 25.degree. C.;
test voltage in parentheses. .sup.b After aging for 200 hr at
100.degree. C. except as noted; test voltage may differ slightly
from that of "As made" samples. .sup.c Based on 50 volt V.sub.t
test except as noted. .sup.d Based on 50 volt V.sub.t test except
as noted. .sup.e Based on 100 volt V.sub.t test. .sup.f Based on 75
volt V.sub.t test. .sup.g After aging for 100 hr at 100.degree.
C.
* * * * *